ILLUMINATING HUMAN HEALTH AND DISEASE

April 2017

Figure 1: Depiction of a mycoplasma (100 times smaller than a human cell), showing how tightly packed it is with proteins and other macromolecules. Illustration by David S. Goodsell, the Scripps Research Institute.

by Dr Liz New

Molecular imaging tools enable us to see inside human tissues and even cells, to understand the chemical factors underpinning biological processes.

Medical imaging techniques, like MRI, PET and CT, now give us very sophisticated information about the structure of organs, tissues, and even cells. But many of the most pressing questions in medical research today are actually on a much smaller scale, and are questions of chemistry rather than biological structure: how chemical (e.g. enzyme) reactions take place; where drug molecules interact with the cell; and how pH changes in health and disease. Traditional structural imaging techniques are no longer sufficient – we now need chemical information to diagnose and understand disease. This requires the use of chemical sensors, often called molecular imaging agents.

There are a number of molecular imaging agents that have already found widespread use. For example, PET scans almost exclusively use 18F-fludeoxyglucose (FDG), a glucose analogue that can therefore report on areas of the body that have highest glucose uptake. However, there remains an abundance of unanswered chemical questions of medical significance, and the development of new molecular imaging agents is an active and exciting research field.

We often think of the cell as a bag, containing mostly water and the odd organelle, DNA molecule, or protein, but this is far from the case. Molecular depictions, like the one of the mycoplasma bacterium shown in Figure 1, illustrate the cluttered environment of the cell. Molecular imaging tools therefore need to find the chemical of interest, and “light up” in some way so that this chemical can be seen over all others. In my research group, we make molecular imaging tools (commonly called sensors or probes) that give out information in the form of fluorescent light (for use in microscopy) or magnetic resonance contrast (for use in MRI).

Figure 2: Our sensor FCR1 gives green fluorescence when oxidised and blue fluorescence when reduced. We can use it to see oxidative stress in cancer cells that are grown in a sphere, to mimic a tumour. By looking at the ratio of green to blue fluorescence, we can see a ring of cells that are exhibiting oxidative stress.

Figure 3: A microscope image of a C. elegans worm illuminated with one of our fluorescent sensors. The round black spots are eggs, which have not taken up the sensor.

One of the main chemical questions we are interested in studying involves oxidative changes in the body. Oxygen is so crucial to our survival, but in high concentrations it can also be highly damaging – it is itself an oxidant, and can produce reactive oxygen species (ROS), many of which are free radicals, which cause damage to DNA, proteins and lipids in the cell. In healthy cells, ROS are balanced by antioxidants to maintain healthy redox homeostasis, with short-lived ROS bursts playing important roles in cellular signalling and immune response. Prolonged elevated levels of ROS are termed “oxidative stress”, which is known to be associated with diseases of ageing, such as cancer, neurodegeneration and cardiovascular disease, but the exact relationships between oxidative stress and disease are not understood. At the other end of the spectrum, lack of essential oxygen, termed “hypoxia”, can also have damaging effects. Hypoxia arises in heart attack and stroke, where there is an insufficient supply of oxygenated blood to the heart and brain, and large cancerous tumours can also be hypoxic. We are therefore developing molecular imaging agents that are capable of detecting hypoxia and oxidative stress.

For studying hypoxia, we are developing magnetic resonance contrast agents. MRI contrast agents are used for over a third of all MRI scans. Contrast agents are typically metal complexes, which improve the overall contrast between bright and dark regions of an MR image. Recently, researchers have tried to make responsive contrast agents, which can report on a particular aspect of the body’s chemistry. We are working with Dr Paul Bonnitcha, an alumnus of the School and now a medical doctor, to develop MR contrast agents that only light up in the presence of hypoxia. Our agents are cobalt complexes, which are less toxic than the commonly-used gadolinium-based MRI contrast agents.

Figure 4: We can send our compounds to specific regions of the cell. Here, we are just imaging the mitochondria of macrophages (a class of white blood cells).

For studying oxidative stress, we have made fluorescent sensors. For example, we have made a sensor, FCR1, which changes from emitting blue light to emitting green light in oxidising environments [Figure 2]. When we look at cells treated with our sensor using a confocal microscope, we can use the ratio of green to blue fluorescence to identify regions that are oxidatively stressed. FCR1 and another one of our sensors are now sold by Stressmarq, a Canadian bioreagents company. We have also sent our sensors to more than twenty research groups around the world, who are using them to answer their own questions about oxidative stress. With collaborators at the Florey Institute in Melbourne, we have used our sensors to image C. elegans, microscopic worms (often called nematodes) [Figure 3].

More recently, we have been developing sensors that localise only to specific parts of the cell, enabling us to specifically understand the chemistry within sub-cellular organelles [Figure 4].

Hypoxia and oxidative stress are just some of the conditions that we are trying to understand better in my research group. We particularly love interacting with medical researchers to learn about what chemical problems they would like to study, and tackling challenges that we know will change our understanding of the human body in health and disease.

CONTACT DR LIZ NEW:

School of ChemistryBuilding F11The University of Sydney NSW 2006Australia